Heat resisting alloys



Oct. 23, 1962- T. A. TAYLOR ETAL HEAT RESISTING ALLOYS Filed June 17, 1957 I0 3 A a C D rm 5 n 5 m m w m Inventor DURATION DAYS.

HEAT RESHSTING ALLOYS Trlstram Allan Taylor and Norman gtephenson, Famham, England, assignors to Minister of Supply, in Her Majestys Government of the United Kingdom of Great Britain and Northern Ireland, London, England Filed June 17, 1957, Ser. No. 666,105 Claims priority, application Great Britain June 21, E56 1 Claim. (Cl. 29-1823) This invention relates to articles made of heat resistant alloys comprising one or more of the metals, nickel, cobalt and iron, which is or are the principal constituent or constituents of the alloy, and also comprising chromium. The invention further relates to methods of manufacturing articles from such alloys.

One class of alloy with which the present invention is concerned is that in which the principal constituent is nickel and which also contains of the order of 20% chromium and a proportion of titanium or proportions of titanium and aluminium. Such alloys may contain a quantity of iron either unavoidably present or consequent upon the use of ferro-alloys to introduce the chromium and other alloying elements. These alloys normally contain minor proportions of silicon, manganese and carbon together with traces of other elements, e.g. copper and sulphur as impurities, and some of them also contain molybdenum and/or niobium. Further, some of the alloys of the class referred to contain no cobalt or only a minor proportion thereof, say up to 4%, but others contain a substantial proportion of cobalt-as much as 15 or 20%. Normally nickel will be the major constituent, exceeding 50%.

The composition of some alloys typical of the class referred to are given in the following table.

Another class of alloys with which the present invention is concerned is that in which the principal constitucut is cobalt and which also contain of the order of 10 to 30% chromium together with varying proportions of many other elements such as iron, carbon, silicon, manganese, nickel, tungsten, molybdenum, niobium and vanadium.

Typical compositions of some alloys of this class are given in the following table.

TABLE II Alloy E, F, G, H, 1,

percent percent percent percent percent 28. 7 29. 6 26. 4 26. 2 18. 7 Trace.-- 16. 2 9.65 12.6 0.28 0.49 0.50 0.48 3.2 6.74 7.44 6. 27 6.16 0.05 2. 25 0.20 0.12 0. 38 0.48 0.32 0.22 0.63 0.39 0. 58 0.91 Maganese 0.76 Nioblurm. 1. 99 Vanadium. 2.

Remain- Remain- Remain- Remainder der der der A third class of alloys with which the present inven- 3,05%,325 Patented Oct. 23, 19232 TABLE III Alloy J, K. L, M, N,

percent percent percent percent percent Chromium 29. l 13. 9 14. 1 Nick 1. 89 13. 6 18. 1 Cobalt 11.7 8. Tungsten. 2. 73 Molybdenum 1. 65 3. 35 Titanium 0. 69 Copper.-- 3. 55 Carbon 0. 42 0. 20 Silicon".-- 0.18 0. 84 Manganese. 0. 78 0.63 Niobium-.. 2. Iron Remain Remain- Remainder der der The lists of alloys given above, are of course, by no means exhaustive, and the specific alloys referred to are given only as examples of the classes of alloys to which the present invention relates.

The invention is also applicable to alloys based on combinations of nickel, cobalt and iron or any two of them. For example, one such alloy contains 20% nickel, 20% chromium, 20% cobalt, 0.3% carbon, 1.5% manganese, 0.6% silicon, 2% tungsten and 3% molybdenum, the balance being iron.

Alloys of the classes referred to above are characterised by their resistance to oxidation at high temperatures and by their good mechanical properties, in particular resistance to creep, and they are accordingly used extensively in gas turbines. However, gas turbine temperatures are still being increased, and it is apparent that there will be a limit of temperature beyond which these alloys will be unsuitable. Moreover, it is found that in gas turbine plant in which certain low grade, e.g. crude and residual, fuel oils, are burnt, these alloys are liable to very rapid corrosion when exposed to the resultant combustion gases even at temperatures at which they would be able to Withstand oxidation in the presence of combustion gases resulting from the combustion of distillate fuels. It appears that this corrosion is due to the presence in the ash of certain deleterious substances, particularly vanadium and sodium compounds such as vanadium pentoxide and sodium sulphate, which attack the oxide film which would otherwise protect the alloy.

The same problem may arise in connection with the heater tubes and furnace components of high temperature steam boilers and possibly other heating plant in which vanadium-bearing fuel oil is burnt.

The object of the present invention is to provide a protective outer layer for articles made of alloys of the classes referred to above, to make it possible to use such alloys both in plant operating at higher temperatures than has hitherto been generally possible, and in plant utilizing low grade fuel oils as mentioned above. Increased resist ance to oxidation can be obtained in some circumstances by the inclusion of beryllium at least in the surface of the article. Thus in the case of a nickel alloy containing 3.5% beryllium, the rate of attack by the fuel ash diminishes with time, eventually becoming negligible. It appears that the vanadium pentoxide in the ash reacts with the nickel or its oxide, ultimately forming nickel vanadate, but the beryllia formed by the oxidation of the beryllium is not attacked. Hence the nickel is extracted 3 from the outer surface of the article, which is thereby enriched in beryllium until a coherent film of beryllia is formed, this film restricting outward diffusion of the nickel IOIIS.

However, with a nickel-3.5% beryllium alloy containing also 20% chromium, such protection against corrosion does not occur so readily. It seems possible that a mixed film of beryllia and chromic oxide is formed which permits the passage of nickel and chromium ions. Thus it is necessary not only to enrich the surface of the article with beryllium but also to deplete it of chromium, which, in the presence of vanadium is unable to form a protective oxide layer.

Accordingly, the invention provides an article made of a heat resistant alloy comprising one or more of the metals nickel, cobalt and iron which is or are the principal constituent or constituents of the alloy, and also comprising chromium, the article having an outer layer containing a chromium-free alloy of beryllium and nickel, cobalt or iron.

Thus in operation the article has a protective outer layer which is less liable to be corroded by vanadium and sodium compounds in the fuel ash, and moreover should enable the article to withstand higher temperatures than would be possible in the absence of such a layer.

The layer may be formed by coating the article with a layer of an alloy containing beryllium and nickel, cobalt or iron but substantially free of chromium, or with a layer of nickel, cobalt or iron only, the surface being subsequently treated to enrich it with beryllium. Alternatively the surface of the article may be first treated to deplete it of chromium and then further treated to enrich it with beryllium. In some circumstances these latter steps may be carried out simultaneously.

A number of methods of carrying out the invention will now be described with reference to the following examples and the accompanying graph which shows the rate of oxidation of metallic specimens in contact wtih vanadium pentoxide.

Example 1 A specimen of alloy B referred to in Table I above, shaped as a solid substantially cylindrical body, /8 inch diameter and weighing 36 gms., with one plain end face and the other end face slightly dished was given a coating 0.003 inch thick of a powdered nickel and beryllium, in proportions corresponding to the nickel-beryllium eutectic containing 5.1% beryllium. The coated specimen was then heated in an electric furnace in vacuo at 1125 C. for 10 minutes. Metallographic examination of the specimen showed that the nickel-beryllium powder had sintered to form a homogeneous adherent layer. The composition of the applied nickel-beryllium alloy may vary within the range to 6 percent beryllium (it may be nickel/5.7 percent beryllium or nickel/6.0 percent beryllium).

A specimen coated in the manner described above was supported on platinum wire at a point on its plain end face with its dished face uppermost in an electric furnace and maintained at a temperature of 700 C. Vanadium pentoxide in powder form was applied to its dished end face intermittently at a rate of 20 mgms. a day, the vanadium pentoxide, being molten at 700 C., spread over the Whole surface of the specimen. The specimen was weighed periodically to ascertain the gain in weight due to oxidation. The results are shown in the accompanying graph in which the ordinate represents the weight gain of the specimen in mgms. and the abscissa the duration of the test in days. The curve A represents the rate of oxidation of the specimen referred to.

For comparison a similar specimen of the same alloy without a protective coating was maintained at a temperature of 700 C. and vanadium pentoxide similarly added intermittently. The curve B represents the rate of oxidation of this specimen and it will be seen that by means of the protective coating the rate of oxidation was substantially reduced.

The experiment was repeated using specimens with protective coatings fused on by heating at 1150 C. for 40 minutes and at 1175 C. for 5 minutes, and the rates of oxidation of these specimens are indicated by the curves C and D respectively. It will be seen that in the case of the specimen having a protective coating at 1175" C. the reduction in the rate of oxidation was less marked, and further experiments indicated that coatings formed at temperatures of 1200 C. and above were generally ineffective.

The effectiveness of the coating in reducing the rate of oxidation was found to be a function of the temperature to which the specimen was heated to fuse on the beryllium-containing coating and the period of time for which it was maintained at this temperature. If the temperature was below about 1100 C. the coating did not adhere adequately to the underlying alloy. On the other hand, if the temperature was too high or the period of heating too long, it was found that chromium from the underlying alloy tended to diffuse outwardly into the coating and thus render it less effective for reducing oxidation. The specimen should accordingly be heated to such a temperature and maintained at that temperature for such a period of time that the coating is only fused or sintered onto the underlying alloy to cause it to adhere firmly thereto and no substantial diffusion of the coating into the alloy or of the chromium into the coating occurs. Thus a temperature of 11001150 C., preferably about 1125 C., and a time of not more than minutes, preferably about 10 minutes is suitable. However in some circumstances it might be possible to heat the specimen rapidly e.g., by induction heating, for a period as short as one minute, in which case a higher temperature, possibly as high as, or higher than, the melting point of the coating, could be employed.

In the experiments described above, the coating was formed from a mechanical mixture of nickel and beryllium in the required proportions to form the eutectic. However it may in some circumstances be preferable to initially melt the mixture to form the eutectic, allow the molten eutectic to solidify, and grind it to form a powder of the eutectic. This procedure should ensure the formation of a homogeneous coating and should promote more rapid sintering or melting thus leading to shorter times of treatment and hence to less outward diffusion of chromium.

It was found that the treatment described had little effect on the creep properties of the specimens. In some circumstances the thickness of the coating may be increased, for example, up to .009 inch.

The thicker coatings may be built up by repeated applications of thinner coatings which are sintered or fused on at each stage.

Example 2 In modifications of the method described in Example 1 above, specimens of the cobalt base alloy I in Table II above and of a chromium-containing iron base alloy were formed with coatings of the nickel-5.1% beryllium alloy referred to. Specimens of these alloys and the nickel base alloy C were also given coatings of an ironberyllium eutectic containing 9% beryllium sintered at 1175 C. and above, and of a cobalt-beryllium alloy containing 4% beryllium sintered at 11001200 C.

Example 3 In a further alternative method, a specimen of alloy B referred to was given a coating of nickel of a thickness of 0.003 inch by an electroplating process. Beryllium powder suspended in xylene was painted onto the surface and the xylene dried off. The specimen was then heated out of contact with air at a temperature of 1200 C. for 2 minutes to diffuse the beryllium into the surface. The

resulting diffused layer could be detected by metallographic methods.

The temperature-duration values are nominal ones for, due to the heat capacity of the furnace, the time during which the specimen was within the temperature range 1150 C.1200 C. would be approximately 30 minutesallowing for the time to heat up and cool down the specimen.

This method was also used with the nickel coating applied by flame spraying it on as a molten powder. A layer of nickel of average thickness 0.005 inch was sprayed on to a specimen of the type described in Example 1, this layer was treated with beryllium in the manner described for the electroplated layer above. On corrosion testing with vanadium pentoxide by the method set out in Example 1, it was found that the rate of oxidation after '10 days was diminished by a factor of 3 compared with the uncoated material. The nickel layer might alternatively be applied by other methods, for example, sputtering, vacuum evaporation, or hot dipping, and other methods of diffusing the beryllium may be used. The coatings might be of iron or cobalt and any of the coatings might be applied to any of the alloys to which this invention relates.

The above methods of carrying out the invention have been described with reference to test specimens, but it will be understood that they are applicable to articles of manufacture such as gas turbine rotor or stator blades.

The methods described in the above examples all involve the application of a coating containing substantially no chromium. In alternative methods described in the following examples, the article or specimen is first treated to deplete its outer surface of chromium and then treated to enrich the surface with beryllium.

Example 4 In this method specimens of alloy C above were heated in air for periods of 16 hours at 900 and 1000 C. A layer of chromic oxide was formed on the surface and this layer was removed. Metallographic examination indicated that an outer layer which was depleted in chromium had been formed on the specimen.

Example 5 In a modification of the method described in Example 4, the specimens were heated in molten caustic soda or caustic potash for various periods, from 1 to 16 hours and at various temperatures from 400900 C. This method should give to the formation of an outer layer depleted in chromium without the necessity for heating for such long periods of time and to such high temperatures as required by the method of Example 4-. In addition this method is thought to involve less risk of intergranular penetration of the underlying alloy.

The methods of Examples 4 and 5 may be applied to articles such as turbine blades made by various methods. Thus the article may be formed by precision forging in known manner and it is found that in this process the surface of the article is depleted of chromium to some extent due to preferential oxidation. Such a blade may undergo a further process in accordance with Examples 4 and 5 to give increased chromium depletion of the surface. Alternatively the article may be formed by some process, such as machining from forged bar, precision casting or powder metallurgy, which does not give rise to such preferential oxidation or at any rate not to such a great extent, and then treated to promote preferential oxidation of the chromium as described above.

An article or specimen having its surface treated as set forth above to deplete it of chromium is then thoroughly cleaned, for example, by shot blasting, vapour blasting, electrolyting descaling, electropolishing, or pickling in acids, or by a combination of these methods, and is then treated to enrich its surface with beryllium. Some methods of treatment are described in the following examples.

Example 6 The article or specimen is heated in contact with powdered beryllium, or a nickel-beryllium, cobalt-beryllium or iron-beryllium alloy out of contact with air, i.e. in vacuo or an inert atmosphere.

Example 7 Beryllium is deposited on the surface of an article by vacuum sputtering or vacuum evaporation, or applied thereto, for example, by painting or spraying in form of a powder in suspension, the article being subsequently heat treated in an inert atmosphere to diffuse the beryllium into its surface.

Example 9 The article is heated in an atmosphere consisting of a volatile beryllium halide and an inert gas such as argon or helium, the partial pressure of the halide being controlled by varying the proportion of inert gas present. Atomic interchange occurs at the surface of the article so that some of the atoms of the nickel or other main constituent or constituents of the article are replaced by beryllium atoms.

Example 10 The article is heated in an air-free vessel in contact with a mixture of beryllium, or a nickel-beryllium, cobalt-beryllium or iron-beryllium alloy or mixtures thereof, an ammonium halide, for example, ammonium chloride, and an inert filler material such as beryllia or magnesia. The ammonium halide dissociates, leading to the formation of beryllium halide which acts in the manner referred to in Example 9 above. Some direct diffusion of beryllium into the blade surface also takes place due to contact with the beryllium-containing mixture.

The surface may be painted or sprayed with beryllium powder in suspension before immersing the blade in a mixture of the filler and ammonium halide, thus achieving an economy in beryllium.

This method may also serve to deplete the surface of chromium, the chromium being removed to the nickelcobaltor iron-beryllium alloy while the beryllium is diffused from the alloy into the blade surface.

Example 11 The article is dipped in a bath of molten nickel-beryllium alloy having a melting point appreciably below that of the alloy of the blade. Thus the alloys A to E referred to above all have melting ranges above 1300 C., and suitable alloys for the bath are nickel-5.1% beryllium With a melting point of -1l57 C. or nickel-25% beryllium with a melting point of 1195 C. Suitable cobaltberyllium or iron-beryllium alloys may also be used.

Example 12 Beryllium may be deposited on the surface of the article electrolytically from a molten salt bath which might consist of a mixture of beryllium fluoride or oxyfluoride with sodium or barium fluoride or oxyfluoride at a temperature high enough to enable beryllium to diffuse into the surface.

It will be understood that while many of the examples are described with particular references to nickel-base alloys, cobalt or iron-base alloys, e.g. the alloys listed in Tables II and III above, may be depleted of chromium and enriched with beryllium by any of the methods already described. It is to be understood that in the methods of Examples 6, 7, and 10 articles made of nickel, cobalt or iron base alloys do not necessarily require the use of nickel-beryllium, cobalt-beryllium and iron-beryllium alloys respectively. Thus an iron-base alloy might be treated with a nickel-beryllium alloy and so on. Similarly when a chromium-free layer of metal is applied as in Examples 1, 2 and 3, the metal applied is not necessarily the same as the base metal of the article.

We claim:

A metal article characterized by high resistance to corrosion by vanadiumand sodium-containing atmospheres that comprises an inner portion consisting of a heat resistant alloy containing, as a major constituent, at least one metal selected from the group consisting of nickel, cobalt and iron and also containing chromium, and a layer being a chromium free alloy of beryllium with a metal selected from the group consisting of nickel, cobalt and iron.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Jaeger: A.P.C. Serial No. 248,647, filed Dec. 30, 1938,

surface layer overlying said inner portion, said surface 15 117/131. 

